The present invention relates generally to optical communications systems, and more particularly, to optical amplifiers.
Optical communication systems typically include a variety of devices (e.g., light sources, photodetectors, switches, optical fibers, modulators, amplifiers, and filters). For example, in the optical communication system 1 shown in FIG. 1, a light source 2, generates an optical signal. The optical signal comprises a series of light pulses. The light pulses are transmitted from the light source 2 to a detector 5. Typically, an optical fiber 4 transmits the light pulses from the light source 2 to the detector 5.
Many optical fibers are lossy in that they scatter (or absorb) portions of light pulses transmitted therealong (about 0.1-0.2 dB/km). When portions of the light pulses transmitted on an optical fiber are scattered (or absorbed), the optical power of such light pulses is reduced. To compensate for optical power losses attributable to the lossiness of optical fibers, optical amplifiers 6 are positioned along the length of the optical fibers 4. The optical amplifiers 6 increase the optical power of the light pulses so light pulses with adequate signal strengths propagate along the length of the optical fiber 4 from the light source 2 to the detector 5.
Optical amplifiers are also useful for transmitting optical signals through free space. Such free-space transmitters are useful for satellite communication links, building-to-building links, intra-city links, ship-to-pier links, and the like. The optical amplifiers provide the high power optical signals (about 100 milliwatts to about 10 watts) needed for transmission across such links.
A cut away view of an optical amplifier 6 useful for optical communication systems or optionally as a free-space transmitter is shown in FIG. 2A. Optical amplifier 6 is a cladding pump optical amplifier. The cladding pump optical amplifier includes a cladding pump fiber 10 and a fiber bundle 20.
The fiber bundle 20 has of a plurality of multi-mode fibers 22 (e.g., 6-19) and a single mode fiber 24. The single mode fiber 24 is positioned at about the center of the fiber bundle 20. The multi-mode fibers 22 transmit pump light. The single mode fiber 24 transmits optical signals.
The plurality of multi-mode fibers 22 and the single mode fiber 24 are fused together into a bundle. The bundle has a diameter which is tapered to match the size and numerical aperture (NA) of the cladding pump fiber 10. The plurality of multi-mode fibers 22 proximate to the tapered end of the fiber bundle 20 are coupled to the cladding pump fiber 10.
A pump diode 25 is coupled to each multi-mode fiber 22 in the fiber bundle 20. The pump diode 25 is coupled to the distal end of the tapered fiber bundle 20. Pump diodes are semiconductor devices designed to emit light at specified wavelengths (e.g., 915 nanometers (nm), 980 nm).
A cross-sectional view of the cladding pump fiber 10 is shown in FIG. 2B. The cladding pump fiber 10 includes a single mode core 12, first cladding 14, and second cladding 16. The single mode core 12 is made of silica that is doped with one or more ionized rare earth elements (e.g., Nd3+, Yb3+, Tm3+, and Er3+). The single mode core 12 typically has a diameter of about 4 xcexcm to about 8 xcexcm. The optical signal is transmitted from single mode fiber 24 to the single mode core 12.
The single mode core 12 is surrounded by first cladding 14. In FIG. 2B, the first cladding 14 is shown with a xe2x80x9cstar-shapedxe2x80x9d cross-section. However, the first cladding 14 optionally has a xe2x80x9crectangularxe2x80x9d (not shown) or a xe2x80x9cD-shapedxe2x80x9d (not shown ) cross-section. First cladding 14 is typically made of silica with an index of refraction suitable for transmitting the pump light.
Pump light from the pump diodes 25 is provided to the first cladding 14 from the multi-mode fibers 22. As the pump light propagates through the first cladding 14, it excites the ionized rare earth elements in the single mode core 12, causing a population inversion. A population inversion is created when more electrons within the ionized rare earth elements are in the excited state than are in the ground state. The energy stored in the inverted population of excited rare earth elements is transferred to the optical signals propagating along the single mode core 12, causing the optical signals to experience an increase in optical power (i.e., a gain). In order to create the population inversion of ionized rare earth elements in the single mode core 12, the wavelength of the pump light must correspond to at least one absorption line for such ionized rare earth element. For example, pump light at 975 nm corresponds to an absorption line of erbium (Er+3).
First cladding 14 is surrounded by second cladding 16. The second cladding 16 is made of a fluorinated, low index of refraction polymer or a low index of refraction glass. The second cladding 16 has an index of refraction that is different from the index of refraction for the first cladding 14. The differences in the indices of refraction for the first cladding 14 and the second cladding 16 substantially confines the pump light within the first cladding 14, preventing it from leaking out of the cladding pump fiber 10. For example, when the index of refraction for the first cladding 14 is about 1.46 and the index of refraction for the second cladding 16 is about 1.38, the difference between the two indices confines about 90% of the pump light in the first cladding.
The electrical efficiency of an optical amplifier is calculated as the ratio of the net optical power output from the amplifier to the power used to operate the pump diodes. The net optical power of the amplifier is defined as the amplifier output power minus the preamplifier power. The power used to operate the pump diodes is defined as the number of diodes times the current-voltage product per diode. For example, when a Er+3/Yb+3 cladding pump fiber pumped with six pump diodes (operated at about 1.7 volts and about 1.5 amps) amplifies a 1550 nm optical signal from a preamplifier power of about 120 mW to an output power of about 1.2 W, the Er+3/Yb+3 cladding pump fiber has an electrical efficiency of about 7% (electrical efficiency=(1.2 Wxe2x80x940.12 W)/(6 diodes x 1.7 voltsxc3x971.5 amps)xc3x97100). This means that only about 7% of the electrical power is used for amplifying optical signals input to the optical amplifier.
In some communications systems (e.g., satellite communication systems), there is a limited amount of electrical power available for system operation. Optical amplifiers with low electrical efficiencies (less than about 10%) are undesirable for use in such communications systems because they consume a substantial portion of the available power, which potentially reduces the power available to operate other devices in the system. Additionally, the cost of some communication systems is directly related to the electrical power needed to operate the devices therein. In particular, the cost of the communications system increases as the electrical power needed to operate the devices in the system increases. Accordingly, optical communication systems that provide greater efficiency are desired.
The present invention is directed to an optical communication system in which a beam splitter is used to direct a portion of the pump light provided to an optical amplifier in an optical communication system to at least one other device in the optical communication system. The beam splitter is configured to receive optical signals as well as pump light. The beam splitter directs a portion of the pump light provided to one optical amplifier in the optical communication system to other devices (e.g., optical amplifiers, filters, modulators) of the optical communication system. Directing a portion of the pump light provided to one optical amplifier in the optical communication system to other devices in the optical communication system conserves electric power by reducing the total electrical power needed to operate such optical communication system.
The optical amplifier has a structure in which a cladding pump fiber is coupled to a fiber bundle. The cladding pump fiber has a single mode core doped with an ionized rare earth element (e.g., Nd3+, Yb3+, Tm3+, and Er3+), a first cladding, and a second cladding. The first cladding surrounds the single mode core, while the second cladding surrounds the first cladding.
Optical signals are transmitted along the single mode core. Pump light is transmitted along the first cladding. The pump light in the first cladding excites the ionized rare earth elements in the single mode core, causing a population inversion among such ionized rare earth elements. Some of the energy stored in the inverted population of excited rare earth elements is transferred to the optical signals transmitted along the single mode core, increasing the optical power thereof.
The fiber bundle includes a plurality of multi-mode fibers and a single mode fiber that are fused together. The single mode fiber is preferably placed at about the center of the fiber bundle. Placing the single mode fiber at about the center of the fiber bundle facilitates the alignment between the single mode fiber and the single mode core when the fiber bundle is coupled to the cladding pump fiber. The first cladding is coupled to a source for pump light via one or more multi-mode fibers in the fiber bundle.
Pump diodes are optionally coupled to one or more of the multi-mode fibers in the fiber bundle. The pump diodes are a source of pump light for the optical amplifier.
Many optical amplifiers have low electrical efficiencies (less than about 10%). Thus, only a small portion of the electrical power provided to the optical amplifier from the pump diodes (less than about 10%) is used to amplify optical signals.
In one embodiment of the optical communication system of the present invention, a beam splitter is coupled to an output of the optical amplifier. The beam splitter is configured to receive both the amplified optical signals and the unused pump light from the optical amplifier. The beam splitter diverts a portion of the unused pump light to at least one other device in the optical communication system for use thereby.
The beam splitter diverts the portion of the unused pump light to other devices in the optical communication system using one or more layers of an at least partially reflective material on a substrate. The one or more layers of the at least partially reflective material preferably reflects light at the wavelength of the pump light. For example, when the pump light has a wavelength of about 975 nm, it is desirable that the one or more layers of the at least partially reflective material be made of a material which reflects light at a wavelength of about 975 nm. Examples of materials suitable for making the one or more layers of the at least partially reflective material include metals, dielectric materials, and polymers. Suitable substrate materials include silicon, fused silica, and quartz.
The beam splitter optionally diverts the portion of the unused pump light to the at least one other device in the optical communication system with patterned layers of the at least partially reflective material on the substrate. Patterning one or more of the layers of at least partially reflective material provides regions on the substrate that are reflective at the wavelength of the pump light as well as regions on the substrate that are not reflective at the wavelength of the pump light. The one or more layers are patterned for example by forming stripes of the partially reflective material on the substrate.
Alternatively, the beam splitter diverts the portion of the unused pump light using micro-mechanical layers. The term micro-mechanical as used in this description refers to at least partially reflective moveable layers on the substrate. When the at least partially reflective layers are micro-mechanical, the diversion of the pump light is controlled by the movement of such layers relative to the substrate. For example, the one or more layers of the at least partially reflective material are moveable relative to the substrate using an electrostatic attraction mechanism. Suitable electrostatic attraction mechanisms include parallel plate capacitive drives and comb capacitive drives.
In an alternate embodiment of the optical communication system of the present invention, the beam splitter is coupled at an input of the optical amplifier. When the beam splitter is coupled at the input of the optical amplifier, a portion of the pump light from one or more of the multi-mode fibers is transmitted to other devices in the optical communication system.
Other objects or features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and do not serve to limit the invention, for which reference should be made to the appended claims.